Hi David,
If you are measuring atmospheric pressure, kPa is the most appropriate SI measure you will get numbers that are approximately 100, if you choose Pa you will get numbers of the order of 100,000. Atmospheres are a historic uni of pressure and is the pressure associated with a "standard atmosphere". It has a value of 101.325 kPa. It has no association with the metric system. The bar is a historic unit based on metric units which was designed to be close to the atmosphere. 1 bar = 100 kPa. It is widely used in industry, for example, car tyre pressure. The millibar is a derivative of the bar, 1 mbar = 100 Pa. Hg and mmHg are the equivalent pressure given by a column of mercury. A standard atmosphere is 760 mm Hg. This means that a column of mercury that is 760 mm high has the same mass as a column of air with ther same cross-section that stretches to the distant end of the atmosphere. It was a common unit of pressure until maybe 50 years ago (and was often an alternative to inche of mercury). It is the defacto unit of pressure used to measure blood pressure. As you say, psi (pounds per square inch) is totally inappropriate, but of the units mentioned, is probably the most useful to visualise what is meant by "pressure". In the case of atmospheric pressure, it means that a column of air with a cross-section of one square inch, weighs N pounds (N = 14.7 in the case of standard atmospheric pressure). Please note that I said "weighs", not "has a mass of", because pounds-weight is a measure of force, not of mass. I trust that this make sense. Regards Martin From: [email protected] [mailto:[email protected]] On Behalf Of [email protected] Sent: 30 June 2014 06:51 To: U.S. Metric Association Subject: [USMA:54077] Pressure Unit While customizing my http://www.TimeAndDate.com preferences today, I became rather curious about why so many options are available for Pressure Unit. I see on the list millabar, Hg, Pa, bars, mmHg, psi, atm and kPa. Now, since I know less about Pressure Units than most of you, I am hoping someone would jump in and recommend the most appropriate one. (I already know that none of you will recommend psi.) Thanks in advance. See attached picture. I hope none of your computers have trouble opening a png picture format. David Pearl www.MetricPioneer.com 503-428-4917
